Life and death in a dynamic environment: Invasive trout, floods, and intraspecific drivers of translocated populations

Abstract Understanding the relative strengths of intrinsic and extrinsic factors regulating populations is a long‐standing focus of ecology and critical to advancing conservation programs for imperiled species. Conservation could benefit from an increased understanding of factors influencing vital rates (somatic growth, recruitment, survival) in small, translocated populations, which is lacking owing to difficulties in long‐term monitoring of rare species. Translocations, here defined as the transfer of wild‐captured individuals from source populations to new habitats, are widely used for species conservation, but outcomes are often minimally monitored, and translocations that are monitored often fail. To improve our understanding of how translocated populations respond to environmental variation, we developed and tested hypotheses related to intrinsic (density dependent) and extrinsic (introduced rainbow trout Oncorhynchus mykiss, stream flow and temperature regime) causes of vital rate variation in endangered humpback chub (Gila cypha) populations translocated to Colorado River tributaries in the Grand Canyon (GC), USA. Using biannual recapture data from translocated populations over 10 years, we tested hypotheses related to seasonal somatic growth, and recruitment and population growth rates with linear mixed‐effects models and temporal symmetry mark–recapture models. We combined data from recaptures and resights of dispersed fish (both physical captures and continuously recorded antenna detections) from throughout GC to test survival hypotheses, while accounting for site fidelity, using joint live‐recapture/live‐resight models. While recruitment only occurred in one site, which also drove population growth (relative to survival), evidence supported hypotheses related to density dependence in growth, survival, and recruitment, and somatic growth and recruitment were further limited by introduced trout. Mixed‐effects models explained between 67% and 86% of the variation in somatic growth, which showed increased growth rates with greater flood‐pulse frequency during monsoon season. Monthly survival was 0.56–0.99 and 0.80–0.99 in the two populations, with lower survival during periods of higher intraspecific abundance and low flood frequency. Our results suggest translocations can contribute toward the recovery of large‐river fishes, but continued suppression of invasive fishes to enhance recruitment may be required to ensure population resilience. Furthermore, we demonstrate the importance of flooding to population demographics in food‐depauperate, dynamic, invaded systems.


2014). As expected for a long-lived (>30 years) organism, long-term mark-recapture studies have
shown adult survival to be high (up to 82% annual survival, Coggins et al. 2006), but variable depending on primary residency in the LCR or Colorado Rivers (61% vs. 78%, Yackulic et al. 2014). Juvenile and young-of-year (YOY) survival can vary dramatically between years, and was thought to be particularly low for fish swept out of the LCR and into the colder Colorado River during monsoon flooding Ryel 1995, Robinson andChilds 2001). Cold hypolimnetic discharge from Glen Canyon Dam favors introduced salmonids, which compete with and prey upon humpback chub , leading to reduced growth and survival in the LCR inflow reach , Yackulic et al. 2018. Yackulic et al. (2014) found high emigration rates of LCR juveniles during monsoon season (July to September) and survival of these emigrants near the Colorado-LCR confluence varied with the abundance of rainbow trout Oncorhynchus mykiss (Yackulic et al. 2018). Warmwater invasive fishes (Marsh and Douglas 1997), or introduced parasites (Campbell et al. 2019) may also threaten humpback chub in the LCR, where survival of all age-classes was surprisingly lower relative to the mainstem (Yackulic et al. 2014). Differences in growth of humpback chub in the LCR and mainstem has been largely attributed to thermal regime differences, whereas the LCR is >6°C warmer (Yackulic et al. 2014), but variation in growth within the LCR may be driven by food availability as well as temperature (Dzul et al. 2017, Stone et al. 2020), and winter flooding may limit growth (Dzul et al. 2016).
Managers were prompted to consider means to establish new spawning populations in tributaries with more benign conditions, including through translocations, following decadalscale declines in abundance of humpback chub (Coggins et al. 2006), persistent threats as described above, and the reliance of the population on reproduction in only the LCR (Valdez et al. 2000). Translocations of humpback chub were first initiated to vacant upstream reaches of the LCR in 2003, where fish remained and grew rapidly (Stone et al. 2020). Building on successes in the LCR, we initiated translocations to Shinumo (2009-2013, Spurgeon et al. 2015b) and Havasu Creeks -2016, Trammell et al. 2012, Healy et al. 2020a; Table S1). Tributaries targeted for translocations are much smaller than others supporting humpback chub populations, but were thought to be suitable to support small populations with fewer threats from invasive fishes (Valdez et al. 2000, Pine et al. 2013. Survival and growth rates in translocated populations in Shinumo and Havasu Creeks were estimated in two prior studies using mark-recapture methods (Spurgeon et al. 2015b, Healy et al. 2020a). While apparent survival (survival confounded by emigration) and individual growth rates in translocated populations were comparable to those of juvenile humpback chub in the LCR, neither study assessed environmental drivers of these vital rates, and study designs were inadequate to estimate true survival. Further, as determined through detections on a passive-integrated transponder (PIT) tag antenna array, nearly half of translocated individuals left in Shinumo Creek within the first year, associated with increasing flow and temperature (Spurgeon et al. 2015b), which potentially limiting the establishment of the population (Pine et 3 al. 2013). The remaining individuals were extirpated from Shinumo Creek during July -August of 2014, following a series of large flood events triggered by intense rainstorms on a fire. In contrast, we observed reproduction and recruitment in the Havasu Creek population, which persists through 2020. Survival of emigrants and fidelity rates are unknown for translocated cohorts due to imperfect detection, and our study aims to quantify these rates, which would not be possible without the inclusion of detections outside release sites (Barker 1997, Schaub andRoyle 2014). Through monitoring conducted by cooperators throughout the Colorado River Ecosystem (CRE; US Geological Survey -Grand Canyon Monitoring and Research Center (GCMRC), unpublished data), defined as the Colorado River and its tributaries in Grand Canyon, and through our own monitoring in the Colorado River adjacent to Shinumo Creek, we have detected individuals that had emigrated from both translocation sites.
Based on these prior studies, we assessed evidence for the following hypothesized relationships between flow, thermal characteristics, and invasive salmonids, and translocated humpback chub population dynamics and individual growth: 1) Individual growth, and recruitment and survival rates will vary with flood frequency, magnitude, timing, and duration. Growth of subadults would be higher during summer months in years with higher frequency of floods that would deliver additional terrestrial diet items (Behn and Baxter 2019) or scour substrates to enhance invertebrate growthof particular importance in Havasu Creek where invertebrate production appears limited relative to other tributaries (Oberlin et al. 1999; Figure S1). While gut fullness in humpback chub was found to be highest during periods of flooding in the LCR (Behn and Baxter 2019), it is uncertain whether the addition of allochthonous food items would translate into higher growth rates. Terrestrial-derived food quality may vary (Brett et al. 2017), and in one translocated population of humpback chub, 4 assimilation of allochthonous diet items was low relative to others (e.g., fish, insects, algae; Spurgeon et al. 2015a). Winter flooding may also limit growth, as in the LCR (Dzul et al. 2016).
We predicted recruitment would be limited during years with higher monsoon flood frequency or intensity, as in the LCR where YOY are transported downstream to the Colorado River (Yackulic et al. 2014). Unlike in the LCR, flood-dispersed juveniles are unable to return to and recruit into translocated populations due to barrier falls near the mouth of each tributary.
Once recruited into the sub-adult or adult population, we would expect minimal effects of flooding on survival, with the exception of ash-laden floods following fires in the Shinumo Creek watershed. Southwestern native fishes vary in their resistance to ash and intense flooding, and a congener was susceptible to ash flows in another Colorado River tributary system (Gido et al. 2019). It is unclear if humpback chub were flushed from Shinumo Creek or suffered high mortality rates during the summer of 2014, but we suspected higher mortality rates occurred.
2) The strength of density-dependence in vital rates was assumed to be greater for juveniles, and weaken with size and age in the LCR by Pine et al. (2013), but Yackulic et al. (2018) found only weak support for density-dependent growth and survival in humpback chub in the Colorado River. Nonetheless, following the onset of reproduction, increasing annual abundance estimates of Havasu Creek humpback chub began to level off, and somatic growth rates were lowest in the largest cohort translocated (2014), suggesting density dependence (Healy et al. 2020a). Therefore, we expect density-dependent growth and recruitment in Havasu Creek, but relationships between density and vital rates may be less important in Shinumo Creek compared to other drivers, given high emigration rates (Spurgeon et al. 2015b), and more abundant food ( Figure S1; NPS unpublished data).
3) We hypothesize rainbow trout would limit growth, survival, and ultimately recruitment in translocation sites. Previous food web analysis showed high trophic niche overlap in Shinumo Creek between rainbow trout and humpback chub (Spurgeon et al. 2015a), suggesting potential competition for food. Higher rates of direct predation by rainbow trout upon YOY or sub-adult humpback chub (cf. Yard et al. 2011) would limit recruitment and survival in years when trout are abundant (Yackulic et al. 2018). Alternatively, trout densities appear to be low based on field observations and catch rates through the duration of our study in Havasu (also see apendix in Healy et al. 2020a) relative to Shinumo Creek, and thus trout may have minimal impact on vital rates in Havasu Creek. While no humpback chub were recovered from rainbow trout stomachs during monitoring in our translocation sites, bite scars were observed, piscivory upon other (more abundant) resident native fishes occurred (Whiting et al. 2014, Spurgeon et al. 2015a, and trout were found to suppress native cyprinid and catostomid distribution and abundance in another tributary (Healy et al. 2020b), suggesting the potential for negative interactions between the two species.